Photosynthesis is a process whereby plants produce their own food. This food that is produced is important for the plant's growth and for organisms that feed off of the plants.
During photosynthesis plants produce glucose and oxygen, glucose is the 'food' for the plants and oxygen is a by-product.
This can be seen in the photosynthesis equation which is:
Carbon dioxide + Water ---light energy---> Glucose + Oxygen
Photosynthesis requires light energy (from the sun usually) to make the reactions happen since light energy is being taken in by the chlorophyll, photosynthesis is an endothermic reaction.
Chlorophyll is located in chloroplasts in plant cells in the palisade cells and spongy mesophyll cells.
Usually, plant leaves are the main organ where photosynthesis takes place but really any part of the plant that is exposed to sunlight can develop chloroplast and photosynthesize.
Plants absorb the carbon dioxide from the air through stomata which allows for gas exchange. And water is absorbed by the roots and transported to the leaves via the xylem.